this is a known problem. We will fix it on the next release.
Meanwhile, simply answer "N" to the Plot the fit [ Y / N ] question. You can plot the output later using sasplot.app or primus.app by clicking on its icon and then opening the output .fit file.

You can also try to call sasplot directly from the OS X command line as:
> open -a /Applications/ATSAS/sasplot.app output.fit
Where output.fit should be the actual name of your output file.
Let me know how it goes,